Method of forming titanium and aluminum seals



United States Patent 3,213,532 METHGD OF FURMING TlTANliUM AND ALUMHNUMSEALS Hamid K. Glaser, Horseheads, and Louis A. Lupica, Elmira, N.Y.,assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., acorporation of Pennsylvania Filed Apr. 3, 1961, Ser. No. 100,293 5Claims. (Cl. 29482) This invention relates to vacuum seals between amember of titanium or a base alloy of titanium and a member of aluminumor a base alloy of aluminum and, also relates to preparing the titaniummember prior to sealing by forming a coating of aluminum thereon.

Although for simplicity only aluminum and titanium are referred to inmost of the discussion hereinafter, it is to be expressly understoodthat the present invention is applicable to joining aluminum or basealloys thereof with titanium or base alloys thereof. Furthermore,aluminum or a base alloy thereof may be used in the coating of thetitanium prior to joining.

In the prior art it is customary in the formation of titanium toaluminum seals to coat the titanium by dipping it into molten aluminumand then joining the aluminum member and the aluminum coated surface ofthe titanium member by soldering or brazing using flux and aluminumsolder. In some applications this method is not satisfactory because thenecessity of a dipping operation and also the use of a flux or the likemakes it difiicult to prevent material from flowing or being splashedover surrounding members.

For example, in electronic tube applications wherein ceramic insulatingmembers are employed, it is essential that they retain their insulatingproperties. Therefore any conducting material which is deposited thereonmust be removed or it substantially impairs the tube characteristics.One such application wherein the present invention is in the formationof titanium to aluminum seals in radiation detectors.

The importance of aluminum and titanium seals in radiation detectorsresults because of the requirement of operating at temperatures of about300 C. or more. Therefore, conventional soft solder seals are notsuitable. Titanium is selected for its ease in joining to insulatingmembers. Both titanium and aluminum have low activation cross-sectionssuitable for use in nuclear devices. The relative expense of titanium isappreciably higher than that of aluminum. Therefore, a joint between adetector cell of titanium and a housing of aluminum is desired.

It is, therefore, an object of the present invention to provide animproved vacuum tight seal between titanium and aluminum and a method ofmaking the same.

Another object is to provide an improved method of forming an aluminumcoating on a titanium member.

Another object is to provide a method of forming seals between titaniumand aluminum without requiring dipping operations or the use of flux.

Another object is to provide a seal between titanium and aluminumsuitable for operation in a radioactive environment at ambienttemperatures up to at least 300 C.

According to one feature of the invention, a method is provided forforming a coating of aluminum on a member of titanium prior to joiningthe members which involves the steps of placing a first aluminum membernear the surface of the titanium member and heating it in an inertatmosphere to a temperature substantially greater then its melting pointto cause it to wet the titanium surface. A seal may then be formed to asecond aluminum member by any suitable method.

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According to another feature of the invention, after the above-mentionedcoating operation is performed, the aluminum coated titanium member isjoined to an aluminum member by the use of an aluminum-silicon alloy asan intermediate. For a close fitting and neat appearing seal, themembers may be machined to allow for the aluminum coating and thealuminum-silicon member between them.

The features of the present invention which are be lieved to be novelare set forth with particularity in the appended claims. The presentinvention, both as to its organization and method of accomplishment,together with the above-mentioned and further objects and advantagesthereof, may best be understood by reference to the followingdescription, taken in connection with the accompanying drawings, inwhich:

FIGURE 1 is a cross-sectional view showing one manner in which atitanium member may be disposed so as to form an aluminum coatingthereon in accordance with this invention; and,

FIG. 2 is a cross-sectional view of a completed titanium-aluminum sealformed in accordance with the present invention.

Referring now to the drawings, there is shown in FIG. 1 a titaniummember 10, having a generally cylindrical configuration, with a ceramicinsulating member 12 therein through which a conductive lead 14 passes.The titanium member 10 is supported on a base 16 and is enclosed by anenvelope 18 of a suitable material such as Vycor which has inlet andoutlet tubulations 20 and 22, respectively, for the flow of an inert gastherethrough.

Around the titanium member it at the end on the base 16 there isdisposed a ring 24 of an aluminum containing material which may bedisposed within a machined notch in the titanium member, which makes aportion 26 of the titanium member have a decreased outer diameter.Around the envelope 18 in the region of the aluminum ring 24 is disposedan RF induction heater 28 to provide heat for the coating of thealuminum onto the portion 26 of the titanium member 10.

In experimental work which preceded the present invention, it was foundthat when the members 10 and 24 were disposed as shown in FIG. 1 andwere heated to an extent that the aluminum melted, there was no wettingof the titanium surface but rather the aluminum would separate from thetitanium and form a molten ring on the base member 16. Therefore, theapparent consequence of these experiments was that this method ofcoating would not be successful. However, further experiments conductedwith the members in the same configuration but with RF induction heatingto a temperature not only sufficient to melt the aluminum ring butconsiderably higher, proved the method was successful. Specifically, itwas found that while aluminum melts at about 660 C. and that no wettingof the titanium surface occurred when the aluminum was merely molten,continued heating to a temperature of about 800 C. or more, as measuredby optical pyrometer, produced a sudden unexpected effect. At thattemperature, the molten aluminum which had flowed away and was actuallyseparated from the titanium member by a distance of about to of an inch,suddenly was caused to flow onto and thoroughly wet the surface of thetitanium member 10 and form a suitable coating thereon for subsequentscaling to an aluminum member.

The reasons for this sudden effect are not fully understood but may bedue in part to the reduction of the aluminum surface tension at thetemperature employed and, also, may in part by due to the sustained RFbombardment at the high temperature which serves to clean the titaniumsurface of any trace of oxide that may be adhering thereto.

It has been found that a minimum temperature of about 800 C. iseffective in practicing this invention. That is, considerably highertemperatures may be employed with success. Temperatures as high as 1200C. have been actually used. However, the higher the temperature the lesscontrol there is in locating the coated material since the wettingaction takes place so fast. Therefore to form a coating which may beaccurately positioned by small movements of the titanium member 10during the course of wetting, temperatures of from about 800 C. to about825 C. are preferred.

Referring now to FIG. 2, there is shown the titanium cylinder 10 afterthe aluminum coating 24 has been applied thereto. The titanium memberwas machined so that the aluminum would fill the notched portion 26.Also shown in FIG. 2 is an aluminum member 30 which is generallycylindrical and has an inner diameter about the same as the outerdiameter of the titanium member 10. The seal is effected between thealuminum 30 and the aluminum coating 24 on the titanium 10 by theinsertion of an aluminum-silicon alloy in the form of a ring 32 whichfits snugly in a machined notch in the aluminum member 30 which causesit to have a portion 34' of increased inner diameter at the sameposition as the portion 26 of diminished outer diameter of the titaniummember. After the members are assembled as shown in FIG. 2 they may beheated in an inert atmosphere to effect sealing.

In the practice of the present invention, the titanium member 10 may beeither pure titanium or a base alloy thereof such as an alloy having thecomposition 6% Al, 4% V and 90% Ti or an alloy having the composition 7%Al, 4% Mo and 89% Ti. The aluminum ring 24 and the aluminum member 30may be either of pure aluminum or a base alloy thereof such as thefollowing (composition in percent maximum unless shown as a range):

posed within a machined notch 34 in the aluminum member 30. The alloymay be the aluminum-silicon eutectic. The joint is then formed by theuse of a tungsteninert gas weld in a manner similar to heliarc welding.

Joints attempted without the use of the intermediate aluminum-siliconalloy 32 are not generally successful because the developement of a pinhole in the seal is a permanent defect which cannot be repaired. The useof the alloy provides a leakproof vacuum joint which can be made byheliarc welding and if pin holes do develop in the joint it is onlynecessary to further heat the seal to close the pin holes.

It is therefore seen that this invention provides a method of forming avacuum seal between titanium and aluminum in a relatively small areawithout using techniques which cause conductive material to be spatteredaround on nearby members. Configurations as shown in the drawing havebeen successfully sealed with the ceramic insulating member 12 remainingfree of conductive matter. Seals formed by the present invention alsoare economical in the use of materials and do not require a high degreeof skill by a human operator.

While the present invention has been shown and described in certainforms only, it will be obvious to those skilled in the art that it isnot so limited but is susceptible of various changes and modificationwithout departing from the spirit and scope thereof.

We claim 'as our invention:

It. The method of forming a coating of aluminum or an aluminum basealloy on a member of titanium or a titanium base alloy comprising thesteps of: placing a first member of a first material selected from thegroup consisting of aluminum and base alloys thereof near the surface ofa second member of a material selected from the group consisting oftitanium and base alloys thereof; and RF induction heating said firstmember in an inert atmosphere to a temperature of at least about 800 C.

Alloy Si Fe Cu Mn Mg Cr Zn Others Al 0.10 0. 01 0.01 0.01 0.10 0. 10 99.45 0.20 0.05 0. 10 0.15 99. 0. 20 1. 0-1. -t 0. 0. Remainder 0 15-0. 40. 15 0. 8-1. 2 0. 15-. 5 0. 0.30 Remainder 0. 10 0. 10 0. -0. 9 0. 100. 10 0.25 Remainder All of the above materials have been successfullyused in the practice of this invention and it is believed clear thatother titanium base alloys and aluminum base alloys are also suitablefor use.

As a specific example of the practice of the present invention, thefollowing steps were performed. A clean titanium member 10 having amachined portion 26 is obtained and a ring of clean aluminum 24 isdisposed about it within the notch as shown in the drawing. The assemblyis supported by any suitable member such as a non-reactive base member16 of a material such as aluminum oxide. Alternatively, the titaniummember 10 may itself support the aluminum ring 24 with the titaniumbeing supported by a suitable fixture. The assembly is disposed in atube of Vycor. An atmosphere of argon or helium is flushed through thetubing to provide a neutral atmosphere, the direction of the flow of theprotective gas being such as to aid in the cooling of the ceramicinsulating member 12. An RF coil 28 around the tubing 18 is located toprovide a concentration of heat in the region of the aluminum ring 24.Heating is continued until wetting of the titanium surface by the moltenaluminum occurs, at a temperature of about 800 C. to about 825 C. Thealuminum coated titanium part is then permitted to cool down to roomtemperature while Within the protective atmosphere. The aluminum coatedtitanium part 10 is then assembled in position with an aluminum member30 with which it is to be joined, as shown in FIG. 2, with a ring 32 ofan aluminum-silicon alloy disto cause said first material to flow uponand to wet the surface of said second member.

2. The method of forming a coating of aluminum or an aluminum base alloyon a member of titanium or a titanium base alloy comprising the stepsof: placing a first member of a first material selected from the groupconsisting of 21 aluminum and base alloys thereof near the surface of asecond member of a material selected from the group consisting oftitanium and base alloys thereof, and heating said first member in aninert atmosphere by RF induction heating to a temperature in the rangeof from about 800 C. to 825 C. to cause it to flow upon and to wet thesurface of said second member.

3. The method of forming a titanium to aluminum seal comprising thesteps of: placing a first member of a first material selected from thegroup consisting of aluminum and base alloys thereof near the surface ofa second member of a material selected from the group consisting oftitanium and base alloys thereof to which the seal is to be made, RFinduction heating said first member in an inert atmosphere to atemperature in excess of about 800 C. to cause said first material toflow upon and to wet the surface of said second member and form acoating thereon, and joining said coated second member to a third memberof a material selected from the group consisting of aluminum and basealloys thereof.

4. The method of forming a titanium to aluminum seal comprising thesteps of: placing a first member of a first material selected from thegroup consisting of aluminum and base alloys thereof near the surface ofa second member of a material selected from the group consisting oftitanium and base alloys thereof to which the seal is to be made, RFinduction heating said first member in an inert atmosphere to atemperature of from about 800 C. to about 825 C. to cause said firstmaterial to flow upon and to form a coating on said second member,placing a third member of an aluminum-silicon alloy in contact with thecoated surface of said second member, placing a fourth member of amate-rial selected from the group consisting of aluminum and base alloysthereof in contact with said third member, and heating said members inan inert atmosphere to a temperature sufficient to join said members.

5. The method of forming a titanium to aluminum vacuum seal comprisingthe steps of: obtaining a first hollow cylinder of a first materialselected from the group consisting of titanium and base alloys thereofhaving a portion of diminished outside diameter at one end thereof,disposing said first cylinder in an envelope containing an inert gasWith a ring of a second material selected from the group consisting ofaluminum and base alloys thereof disposed around said first cylinder onthe portion of diminished outside diameter, heating said ring by RFinduction heating from about 800 C. to about 825 C. to cause said secondmaterial to flow upon and to form a coating on the surface of theportion of diminished outside diameter of said first cylinder,permitting said first cylinder to cool to room temperature, obtaining asecond hollow cylinder of a material selected from the group consistingof aluminum and base alloys thereof having a portion of increased innerdiameter at one end thereof, placing said second cylinder around saidfirst cylinder so said portion of increased inner diameter faces thecoated surface of said first cylinder, placing a member of a eutecticalloy of aluminum and silicon in the space between the portion ofincreased inner diameter of said second cylinder and the coated surfaceof said cylinder, and heating in an inert atmosphere to bond saidmembers together.

References Cited by the Examiner UNITED STATES PATENTS 2,528,758 11/50King 219--9.5 2,925,647 2/57 Jones et al 29-527 X 2,992,135 7/61 Finlay29-197 X 3,010,198 11/61 Hanink et al. 29527 OTHER REFERENCES ProductEngineering, February 1943, page 103. Iron Age, July 11, 1946, page 54.

JOHN F. CAMPBELL, Primary Examiner.

1. THE METHOD OF FORMING A COATING OF ALUMINUM OR AN ALUMINUM BASE ALLOYON A MEMBER OF TITANIUM OR A TITANIUM BASE ALLOY COMPRISING THE STEPSOF: PLACING A FIRST MEMBER OF A FIRST MATERIAL SELECTED FRM THE GROUPCONSISTING OF ALUMINUM AND BASE ALLOYS THEREOF NEAR THE SURFACE OF ASECOND MEMBER OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OFTITANIUM AND BASE ALLOYS THEREOF; AND RF INDUCTION HEATING SAID FIRSTMEMBER IN AN INERT ATMOSPHERE TO A TEMPERATURE OF AT LEAST ABOUT 800*C.TO CAUSE SAID FIRST MATERIAL TO FLOW UPON AND TO WET THE SURFACE OF SAIDSECOND MEMBER.
 3. THE METHOD OF FORMING A TITANIUM TO ALUMINUM SEALCOMPRISING THE STEPS OF: PLACING A FIRST MEMBER OF A FIRST MATERIALSELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND BASE ALLOYS THEREOFNEAR THE SURFACE OF A SECOND MEMBER OF A MATERIAL SELECTED FROM THEGROUP CONSISTING